A photoprocessing technique that utilizes a photosensitive material has played an important role in industry. The greatest advantage of such a photoprocessing technique is that a fine pattern can be quickly formed in a contactless manner. For example, a photosensitive material that changes in solubility or the like by light exposure (photoresist) is used for plate making in the field of printing or for microfabrication in the field of electronics, wherein a printing pattern or a circuit pattern is formed by the presence or absence of light exposure.
However, since an existing material normally utilizes an irreversible photoreaction (polymerization or decomposition), it is difficult in principle to restore the material to its original state after light exposure. Actually, the material has been disposed of after use. Therefore, development of a photosensitive material that can be used repeatedly has been one of the most important subjects for green innovation that implements energy conservation and resource saving.
A photoisomerization reaction has been known as a reversible (repeatable) photoreaction, and azobenzenes have been known as typical compounds that can undergo a photoisomerization reaction. The molecule of azobenzenes changes from the trans-configuration to the cis-configuration (isomer) by exposure to ultraviolet light, and restores to the trans-configuration by exposure to blue light or by placement in a dark place. In principle, this reaction (photoisomerization reaction) can repeatedly occur any number of times.
However, it has been known that the reaction easily occurs in a solution state, but rarely occurs in solid (both in pure substance and in mixture). The reason is considered that a free volume is insufficient in solid where molecules are densely packed. The term “solid” used herein refers to both amorphous solid and crystalline solid.
In recent years, deformation of crystals along with photoisomerization (Non-patent Document 1) and photoisomerization of microcrystals (Non-patent Document 2) have been reported as rare examples of photoisomerization of azobenzenes in crystals. However, a change from a solid state to a liquid state due to photoisomerization has not been reported. Non-patent Document 3 and Non-patent Document 4 propose diarylethenes as compounds that undergo a reversible photoreaction in crystals. However, it has not been reported that this molecular system undergoes solid-liquid phase transition.
In view of the above situation, it has been desired to develop a technique that improves the photoreactivity of azobenzenes in solid and that dramatically changes the properties (solid/liquid) of azobenzenes, in order to apply the photoisomerization reaction of azobenzenes to a solid photosensitive material.
Regarding compounds in which a plurality of azobenzenes are bonded to form a ring (macrocyclic azobenzenes), Non-patent Documents 5 to 10 suggest that the compounds differ in photoreactivity from normal azobenzenes due to a special environmental field caused by the ring structure and that the shape of the molecule (i.e., the shape of the ring) changes from a planar shape to a significantly distorted shape along with a photoisomerization reaction.
The inventor of the present invention found and reported a photo-induced melting phenomenon in crystals of compounds which have a long-chain alkoxy group that is radially introduced into the cyclic skeleton of a dimer or a trimer of macrocyclic azobenzenes (Patent Document 1 and Non-patent Document 11). Specifically, the inventor found a phenomenon in which the compounds (azobenzene derivatives) change into liquid by exposure of solid (crystals) to light and the produced liquid restores to solid when heated. The azobenzene derivatives are characterized in that they can repeatedly undergo phase transition between a solid state and a liquid state any number of times.
Non-patent Document 12 suggests stilbene derivatives as compounds that change from liquid into solid due to a photoisomerization reaction. However, the stilbene derivatives do not change from solid to liquid by exposure to light.